This invention relates to composite solid propellants and in particular to composite propellants for rocket catapult motors.
Composite solid propellants are mixtures of a finely ground oxidizer and a solid fuel dispersed in a matrix of plastic, resinous, or elastastomeric material. These propellants can also contain other compounding ingredients in order to change the ballistic or physical properties of the propellant.
The correlation between the burning rate and the combustion chamber pressure is commonly expressed as r=Kp.sup.n or log r=n log P + log K where r is the burning rate in inches per second; P is the combustion chamber pressure in pounds per square inch; K is a constant which varies with the ambient grain temperature, and n is a constant known as the burning rate exponent. Ideally a plot of log r against log P would give a straight line with a slope of n for a non-modified propellant.
From frequent usage, the value of the burning rate exponent have become an accepted measure of the pressure sensitivity of a propellant. The sensitivity of the burning rate to changes in the comubstion chamber pressure decreases as the value of n approaches zero. In compounding a propellant, it is extremely important that the value of n be kept as low as possible. If n exceeds 0.95, the combustion reaction becomes uncontrolled and the combustion chamber explodes. Values of n between 0.75 and 0.95 require great care in selecting the charge geometry and nozzle design. Even with those precautions, there is little likelihood that the combustion reaction would be controllable because unfortunately the burning rate exponent does not remain constant. If the burning rate exponent stays below 0.75, the burning is managable. Thus, any increase in n simply causes the pressure and the burning rate to increase.
At some pressure, composite propellants will experience a large and sudden increase in the burning rate exponent over a small increment in pressure. For example the exponential will jump from 0.5 to 0.9 with a small increase in pressure. The pressure at which the burning rate exponent begins to change radically is termed the critical pressure.
The value of this pressure is of paramount importance in compounding high thrust propellants. It is often considered the chief limitation of this type of propellant because if the burning rate remains insensitive to combustion chamber pressure at higher pressures, there is greater latitude in choosing the charge geometry or the nozzle design. In military applications, this is further compounded by the requirement that the propellant have a consistant performance over an ambient temperature range of -65.degree. F to + 165.degree. F. As the ambient temperature increases, the burning rate, the combustion chamber pressure, and the burning rate exponent increases.